Electrical power line owners have a desire and need to transmit more power over existing power lines. As the amount of power being transmitted over a given line increases, the temperature of the conductor increases. As the temperature of the conductor increases, the sag of the conductor increases. Existing weather conditions such as solar heat, ambient air temperature, wind and heat radiation also affect the amount of sag and swing of the line. Excess sag can put the conductor so close to objects on the earth as to cause flashover, which can damage the line and nearby facilities, cause power outages and endanger animal and plant life. High winds can also act to blow a line sideways, dangerously close to or over the edge of the power line right of way.
This device allows the operator to know the actual position of the conductor without having to measure line temperature, tension or angle, nor wind and air conditions along the line. Knowing the actual position of the line relative to the ground and right of way boundaries allows the line operator to increase or decrease the power transmission appropriately so as to attain the maximum flow of power without violating safety clearances.
Moreover, present calculations of expected blow-out under a given set of conditions of line temperature, wind speed, span length, and such give results in excess of those actually observed. Combining this device with devices to measure environmental conditions such as air temperature, wind speed and direction and solar radiation can assist in developing accurate calculations that more closely match actual observations.
Traditionally, transmission lines were rated based on an assumed combination of worst cooling conditions, consisting of a combination of expected highest ambient temperature, solar radiation and a low wind speed. Such traditional current ratings are highly conservative. To take advantage of this conservatism, methods have been developed to either monitor some of the cooling conditions or the actual temperature of the conductor, and to adjust current ratings based on such monitored data.
In the prior art, a number of methods have been used for estimating line position by estimating its temperature. In one such method, using theoretical calculations, assumptions are made of wind speed and direction, ambient temperature and solar radiation. Calculations are made for arriving at the line condition. Because the calculations are based on theoretical assumptions, the result can be at considerable variance from the actual line condition which might permit greater line current than exists or, on the other hand, mandate a lower actual line current.
In some instances weather stations have been established in the general location of the transmission line in order to monitor the weather to thus provide somewhat more reliable data to calculate the line conditions, including the temperature of the line, and thus its actual position.
A third method for monitoring the line known to the prior art is to provide sensor devices mounted on the conductor along the length of the line at various intervals to measure conductor temperatures, from which load capacity can be determined. These various systems of the prior art are disclosed in the U.S. Pat. Nos. 4,268,818 and 4,420,752 and 4,806,855. These monitors have been somewhat more effective in identifying actual temperatures of the conductors. However, because the sensor modules are mounted on the energized conductor, the manufacturing and installation cost of the sensors is complicated and expensive.
A further disadvantage of these conductor temperature based rating methods is that they cannot take into account the progressive stretching of conductor (creep), caused by variation of conductor loading. Design sag and tension tables of conductors are available to help determine the conductor tension and sag in initial condition (before any creep) and final condition (after calculated maximum creep). However, the resulting uncertainty between the sags can be more than 10% of sag, and equivalent to a temperature uncertainty of 25 to 30 degrees Celsius.
In another method in current use, devices can be installed on the line to monitor the tension in the conductor. Conductor tension is combined with other environmental data, such as the ambient temperature and solar radiation, to predict the rating or electrical capacity of the line. U.S. Pat. Nos. 5,517,864 and 5,235,861, relate to methods of calculating the approximate actual sag of an overhead power transmission line by measuring the amount of tension on the line at `dead-end` structures, either by tensiometers or swing angle indicators, as well as measuring ambient temperature, both done at two different times, with no power flow, and then remotely transmitting that information to a computer for performance of theoretical calculations. From the data received, a Ruling Span can be calculated from which to determine a maximum safe current that can be transmitted by the existing line without creating excess conductor sag. These methods require removing the line from service on at least two different occasions for a period of time long enough to allow the conductor temperature to reach ambient air temperature.
Israel Electric Company, Haifa, Israel has occasionally used selected algorithms to approximate actual sag under certain combinations of conditions of weather, power transmission and physical design; using basic longitudinal load modeling techniques.
Power Line Systems, Inc. is understood to have a computer program that performs a mathematical analysis of sag and tension, using longitudinal load modeling, including allowance for longitudinal insulator movement.
All of these uncertainties are eliminated by knowing the actual position of the line with real-time measurement.
Methods exist for determining the sagging accuracy by observations. The traditional method has been based on daytime observations of the sag of the line using optical devices, such as transits, or by observing the propagation velocity of transverse vibration of the conductor (bounce method). Such observations are, of necessity, made during daytime. Sag measurements using such techniques are subject to two main error sources: 1) If the line carries a current, the temperature of the conductor cannot be accurately estimated, and 2) even if the line is de-energized, the solar radiation on the conductor can cause up to a 15degree C. temperature rise compared to the ambient, as noted in U.S. Pat. No. 5,517,864.